1. Field of the Invention
The present invention relates to a device which controls the displacement of a hydraulic pump and a brake control device which controls to brake a hydraulic motor by controlling the displacement of the hydraulic pump, and more particularly to a device which is applied to the hydraulic pump which is not provided with a feedback servo valve.
2. Description of the Related Art
The displacement of the hydraulic pump can be controlled by two methods. One of them is a method to control the displacement by feeding back the present displacement of the hydraulic pump by the feedback control valve. It is called a servo method.
The other is a direct control method which is not provided with a feedback control valve. Operation of a hydraulic pump of the direct control method will be described with reference to the hydraulic circuit of FIG. 17.
FIG. 17 shows a closed circuit which is comprised of hydraulic pump 11 and hydraulic motor 12. For example, it is assumed that a vehicle is driven by rotating crawler belts (or wheels) 13 by the hydraulic motor 12.
When operation lever 14a is operated, a pilot pressure oil with a pilot pressure corresponding to the operated amount is supplied to piston 15 for controlling the displacement. The pilot pressure oil is supplied by pilot pump 16 used as oil pressure source. The displacement control piston 15 is connected to a swash plate, namely locker cam 11a, of the hydraulic pump 11. The displacement control piston 15 moves to a position according to the pilot pressure. When the displacement control piston 15 moves, the swash plate 11a of the hydraulic pump 11 is tilted. The hydraulic pump 11 is changed to have a displacement corresponding to the operated amount of the operation lever 14a. The pressure oil delivered from the hydraulic pump 11 is delivered to the hydraulic motor 12, and the hydraulic motor 12 is driven to rotate. When the hydraulic motor 12 is driven to rotate, the crawler belts 13 connected to shaft 12a of the hydraulic motor 12 are rotated.
The hydraulic pump 11 is easily influenced by a tilting moment because it does not have a feedback servo valve. This function that the hydraulic pump 11 is influenced easily by the tilting moment is used to control pump absorption torque and to control for preventing a shock involved in starting or stopping motion of a vehicle.
In the hydraulic pump 11 of the direct control method, the tilting moment corresponding to the pump pressure acts on the locker cam to change the pump displacement. This is achieved by a shape of a delivery port of the valve plate of the hydraulic pump 11.
The hydraulic pump 11 for a closed circuit has a port shape allowing to deliver from both delivery ports 11b, 11c. This port shape is suitable for delivering a high pressure oil but not suitable for sucking a low pressure oil. To raise the pressure of the suction side of the hydraulic pump 11, it is necessary to make supplemental supply of the pressure oil to the suction side port of the hydraulic pump 11. It is not illustrated but for example a charge circuit for discharging the pressure oil of about 3 MPa from the charge pump is generally disposed. The pressure supplied to the hydraulic pump 11 is sufficient in about 1 to 1.5 MPa. But, a charge pump for discharging a pressure of about 3 MPa is used because the charge pump is often used also as an oil pressure source of the pilot circuit.
The charge circuit is comprised of the charge pump, a charge relief valve for setting a charge pressure, and two sets of suction valves (check valves) for supplying the charge pressure to a low-pressure side port of the hydraulic pump 11, namely to a low-pressure side oil passage.
The closed circuit shown in FIG. 17 is provided with two sets of safety valves in order to prevent equipment from being broken resulting from an excessive increase in pressure within the oil passage. A suction safety valve which has the safety valve and the suction valve (check valve) combined into one body is often used.
It is assumed that a vehicle is started forward. When the vehicle starts to move, the pressure oil is sucked from oil passage 10R into the port 11b of the hydraulic pump 11 and then delivered from the port 11c to oil passage 10F. The delivery side oil passage 10F has a pressure higher than the intake side oil passage 10R. It is when the hydraulic pump 11 is pumping. The hydraulic pump 11 is designed to have a characteristic that the swash plate 11a is returned toward a neutral position as the delivery side oil passage 10F has a higher pump pressure. In other words, the tilting moment corresponding to the pump pressure on the delivery side acts on the locker cam to return the swash plate 11a to the neutral position, so that the displacement of the hydraulic pump 11 becomes small as the pump pressure on the delivery side becomes high.
Thus, when the vehicle starts to move and the delivery pressure increases, the pump displacement can be reduced. Specifically, the pump displacement to an instruction value is delayed from changing to the maximum side to prevent the pump pressure from increasing sharply, thereby limiting the torque output by the hydraulic motor 12. Accordingly, the shock caused when the vehicle starts to move can be lowered.
The same is also applied to a case of stopping the vehicle.
When the vehicle is to be stopped, the oil passage 10R on the intake side has a pressure higher than that of the delivery side oil passage 10F. It is when the hydraulic motor 12 is pumping. The hydraulic pump 11 is provided with a characteristic that the swash plate 11a is raised to the maximum displacement side as the suction side oil passage 10R has a higher pump pressure. Specifically, the tilting moment according to the intake side pump pressure acts on the locker cam to raise the swash plate 11a to the maximum displacement side, so that the hydraulic pump 11 has a larger displacement as the intake side pump pressure becomes higher.
The operation when the vehicle is braking will be described specifically.
To apply the hydraulic brake to the hydraulic motor 12, a flow rate which the hydraulic pump 11 sucks in through the port 11b can be reduced with respect to the flow rate that the hydraulic motor 12 discharges.
When the operation lever 14a is returned to the neutral or the brake pedal is depressed while the vehicle is moving forward, the piston 15 is about to return to the neutral by the spring force.
Thus, the pressure oil flowing through the oil passage 10R is stopped by the hydraulic pump 11 and its pressure is increased to brake the hydraulic motor 12. When the pressure of the oil passage 10R is increased, the displacement of the hydraulic pump 11 is increased by the tilting moment, and the flow rate taken in by the hydraulic pump 11 through the port 11b is increased. Therefore, the pressure oil which was stopped by the hydraulic pump 11 flows out, and the pressure becomes. A braking torque is prevented from becoming excessive by the above operation.
If braking is effective, the speed of the vehicle lowers, and the rotating speed of the hydraulic motor 12 lowers, too. When the rotating speed of the hydraulic motor 12 lowers, the flow rate discharged from the hydraulic motor 12 is decreased, and a brake pressure decreases. When the brake pressure lowers, the tilting moment becomes small, and the piston 15 can be moved in the neutral direction by the spring force. Here, the flow rate to be taken in becomes smaller than that discharged from the hydraulic motor 12, and the brakes are applied continuously.
The piston 15 gradually returns to the neutral while keeping such a balance.
When the piston 15 has returned to the neutral while keeping the balance, it means that the flow rate discharged by the hydraulic motor 12 has become zero and the vehicle has stopped.
Thus, when the suction side pressure (brake pressure) rises and the braking torque increases while the vehicle is braking, the pump displacement can be increased, so that the braking torque can be decreased. In other words, the pump displacement to the instruction value is delayed from becoming minimum, so that the braking torque can be prevented from becoming excessive, and a shock caused while the vehicle is braking can be decreased.
Japanese Patent Laid-Open Publication No. 10-184906 describes an invention related to a hydraulic pump of direct control method.
On the other hand, a hydraulic pump provided with a feedback servo valve has a displacement of the same size as that instructed by the operating lever 14a. Therefore, a separate modulation circuit or the like is necessary to reduce a shock. And, a separate absorption torque control valve or the like is necessary to limit the torque.
But, sometimes a characteristic of the hydraulic pump of the direct control method is an obstacle to the braking of the vehicle.
Specifically, while the vehicle is braking, the pump displacement becomes large and the braking torque becomes small as the suction side pump pressure (brake pressure) increases. Therefore, it takes time to return the displacement of the hydraulic pump 11 to the neutral position (displacement zero), resulting in a disadvantage that braking time and braking distance become long. In other words, when it is necessary to make an emergency stop, there was often a problem that the displacement of the hydraulic pump did not return to the neutral position with good responsivity.
The present invention has a first object to change the displacement of a hydraulic pump of the direct control method to a desired displacement quickly with good responsivity by changing the displacement of the hydraulic pump to a desired displacement forcefully according to an instruction given independent of a usual instruction.
There are two ways to brake the hydraulic motor 12. One of them is to stop the hydraulic motor 12 by setting the displacement of the hydraulic pump 11 to a neutral position to cut off the pressure oil discharged from the hydraulic motor 12 so to operate the hydraulic brakes. A discharge pressure of the hydraulic motor 12 is increased, so that a braking force is applied to the hydraulic motor 12. It is called a hydraulic brake. The other method is to stop the hydraulic motor 12 by fixing the shaft 12a of the hydraulic motor 12 by a frictional force of braking member 17. It is called a mechanical brake.
FIG. 18 shows a brake characteristic when driving down a hill. The horizontal axis indicates elapsed time t after giving a braking instruction. And, the vertical axis indicates a vehicle speed.
In FIG. 8, A indicates a characteristic of braking by the hydraulic pump of a conventional direct control method.
According to the direct control method, the hydraulic brake takes a long braking time as described above. Therefore, the braking time is decreased by operating the mechanical brake at time t1 while the hydraulic brake is operating.
But, there is a problem that a heat load become high because the shaft 12a of the hydraulic motor 12 rotating at a high speed is forcedly fixed by the braking member 17. Besides, a large-capacity mechanical brake is needed to stop the shaft 12a of the hydraulic motor 12 rotating at a high speed. Therefore, there is a drawback that the mechanical brake becomes large in size.
The mechanical brake is generally built in a housing of the hydraulic motor 12.
When the mechanical brake is operated while the shaft 12a of the hydraulic motor 12 is rotating at a high speed, wear powder produced by friction is dispersed within the motor housing. Therefore, the parts such as a sealing material in the motor housing are adversely affected, possibly resulting in deteriorating durability of the hydraulic moor 12.
It is a second object of the invention to improve the durability to prevent a heat load from increasing and to reduce the displacement of the mechanical brake by operating the mechanical brake after finish operating the hydraulic brake.
Besides, it is a third object of the invention to further reduce the displacement of the mechanical brake by forcedly changing the displacement of the hydraulic pump 11 to the neutral position according to an instruction for emergency braking to apply the hydraulic brake quickly with good responsivity and operating the mechanical brake after finish operating the hydraulic brake.
In order to achieve the first object, a first aspect of the invention is a displacement control device for a hydraulic pump which is provided with a displacement control valve (8) for changing a volume of pressure oil delivered from a hydraulic pump (11) in response to a first instruction signal, and controls the volume of pressure oil delivered from the hydraulic pump (11) by entering the first instruction signal into the displacement control valve (8), wherein:
displacement control means (21, 23, 6) are disposed to control to discharge from the hydraulic pump the pressure oil in a volume corresponding to a second instruction signal regardless of the input of the first instruction signal (11) by entering the second instruction signal different from the first instruction signal.
The first aspect of the invention will be described specifically with reference to FIG. 1.
According to the first aspect of the invention, when emergency brake switch 21 is turned on, emergency brake signal S2 is entered control valve 23 for emergency brake. Thus, an emergency brake signal pressure is entered piston 8 through emergency brake signal oil passage 6, and the piston 8 is forcedly positioned at neutral position 8c. In other words, the piston 8 can be forcedly positioned at the neutral position 8c according to the emergency brake signal S2 regardless of the input of usual brake signal S1. Therefore, the displacement of the hydraulic pump 11 can be forcedly set to the neutral position (minimum displacement).
As a result, when it is desired to stop a vehicle urgently, the displacement of the hydraulic pump 11 can be returned to the neutral position with good responsivity, and the vehicle can be stopped urgently according to an operator""s intention.
Besides, even when the hydraulic pump 11 of the direct control method is used, the displacement of the hydraulic pump 11 can be forcedly changed to a desired displacement according to an instruction from another channel different from an ordinary instruction, so that there is obtained an effect that the hydraulic pump 11 can be changed to a desired displacement quickly with good responsivity.
In order to achieve the second object, a second aspect of the invention is a brake control device for a hydraulic motor which is provided with a displacement control piston (8) for changing a volume of pressure oil discharged from a hydraulic pump (11), a hydraulic motor (12) which is driven by the pressure oil supplied from the hydraulic pump (11) and brake means (17, 18) which brake a shaft (12a) of the hydraulic motor (12), controls the volume of pressure oil discharged from the hydraulic pump (11) to control the drive of the hydraulic motor (12) and also to control the braking by the brake means (17, 18), wherein:
brake control means (32, 36) are disposed to control the displacement control piston (8) to discharge the pressure oil in a minimum volume from the hydraulic pump (11) by entering a brake instruction signal and also to control the brake means (17, 18) to the brake the shaft (12a) of the hydraulic motor (12) after the displacement of the hydraulic pump (11) becomes minimum.
The second aspect of the invention will be described specifically with reference to FIG. 4.
According to the second aspect of the invention, the piston 8 operates according to usual brake signal S1 to position at neutral position 8c, and the displacement of the hydraulic pump 11 becomes minimum. It is detected by neutral signal detection oil passage 32 that the displacement of the hydraulic pump 11 has became minimum, and a neutral signal (pressure zero) is entered the control valve 36. Thus, the control valve 36 is positioned at brake release position 36b. Therefore, the pressure oil is discharged from cylinder chamber 18a of the brake cylinder 18 to operate the brake member 17, and the shaft 12a of the hydraulic motor 12 is fixed by the brake member 17. Thus, the shaft 12a of the hydraulic motor 12 can be braked after the displacement of the hydraulic pump 11 has become minimum.
According to the second aspect of the invention, the mechanical brake can be operated after the hydraulic brake is operated without fail, so that durability of the hydraulic motor 12 can be improved and a heat load can be prevented from increasing. And, the capacity of the mechanical brake can be reduced.
To achieve the third object, a third aspect of the invention is a brake control device for a hydraulic motor which is provided with a displacement control piston (8) for changing a volume of pressure oil discharged from a hydraulic pump (11) according to a first instruction signal, a hydraulic motor (12) which is driven by the pressure oil supplied from the hydraulic pump (11) and brake means (17, 18) which brake a shaft (12a) of the hydraulic motor (12), which controls the volume of pressure oil discharged from the hydraulic pump (11) by entering the first instruction signal into the displacement control piston (8) to control the drive of the hydraulic motor (12) and also to control the braking by the brake means (17, 18), wherein:
brake control means (21, 23, 6, 32) are disposed to control the displacement control piston (8) so to discharge the pressure oil in a minimum amount from the hydraulic pump (11) regardless of the entry of the first instruction signal by entering a brake instruction signal different from the first instruction signal and also to control the brake means (17, 18) so to brake the shaft (12a) of the hydraulic motor (12) after the displacement of the hydraulic pump (11) becomes minimum.
The third aspect of the invention will be described specifically with reference to FIG. 2.
According to the third aspect of the invention, when the emergency brake switch 21 is turned on, emergency brake signal S2 is entered the emergency brake control valve 23. Thus, an emergency brake signal pressure is entered the piston 8 through the emergency brake signal oil passage 6, and the piston 8 is forcedly positioned at the neutral position 8c. In other words, the piston 8 can be forcedly positioned at the neutral position 8c according to the emergency brake signal S2 regardless of the entry of the usual brake signal S1. Therefore, the displacement of the hydraulic pump 11 can be forcedly set to the neutral position (minimum displacement). It is detected by the neutral signal detection oil passage 32 that the displacement of the hydraulic pump 11 has become minimum, and a neutral signal (pressure zero) is entered oil passage 33. Thus, when a predetermined time has passed after the neutral position was detected, the pressure oil is discharged from the cylinder chamber 18a of the brake cylinder 18 to operate the brake member 17, and the shaft 12a of the hydraulic motor 12 is fixed by the brake member 17. Thus, the shaft 12a of the hydraulic motor 12 can be braked after the displacement of the hydraulic pump 11 has become minimum.
According to the third aspect of the invention, the displacement of the hydraulic pump 11 is forcedly changed to the neutral position according to the emergency brake instruction, and the hydraulic brake can be effectively applied quickly with good responsivity. Besides, the mechanical brake is operated after the hydraulic brake has operated without fail, so that the capacity of the mechanical brake can be reduced.